The present invention relates to a method of controlling the air-fuel ratio of an internal combustion engine, and more particularly, to a method of this kind wherein the air-fuel mixture supplied to the engine is feedback-controlled to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics in approximate proportion to the exhaust gas ingredient concentration.
Among conventional methods of feedback-controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine (referred to hereinafter as "supply air-fuel ratio") to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics proportional to the exhaust gas ingredient concentration, there is a method proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 62-251443, in which a proportional term (P term), an integral term (I term), and a differential term (D term) are calculated based on a difference between an actual air-fuel ratio detected by the exhaust gas ingredient concentration sensor and a desired air-fuel ratio, and by the use of these calculated P, I, and D terms the supply air-fuel ratio is feedback-controlled.
However, according to this conventional method, feedback gains applied to calculation of the P, I, and D terms are set based on the engine rotational speed and the difference between the actual air-fuel ratio and the desired air-fuel ratio, but not set with other operating parameters of the engine taken into consideration. Therefore, the proposed method has the following disadvantage:
When the engine is in a predetermined accelerating condition, supply of an increased amount of fuel suitable for the accelerating condition (hereinafter referred to as "increased fuel supply for acceleration") is carried out. During increased fuel supply for acceleration, the actual air-fuel ratio detected by the exhaust gas ingredient concentration sensor is deviated toward the richer side relative to the desired air-fuel ratio. However, if the amount of fuel supplied to the engine is rapidly decreased in response to this deviation, the supply air-fuel ratio is largely deviated in a leaning direction immedidately after termination of increased fuel supply for acceleration, which results in degraded driveability of the engine.